There are existing different systems capable of turning a lighting load on if a pyroelectric sensor detects a human body.
Such systems however have an issue that a lighting load is turned off if a human does not move, because such a pyroelectric sensor cannot detect a human in an immobile state.
The issue can be solved by a human body detecting device disclosed in Japanese Patent Application Publication Number 11-37840 (hereinafter referred to as a “document 1”). The human body detecting device is provided with a shutter which is located between a human and a pyroelectric element and is configured to start closing and opening after human movement is detected. According to the device of the document 1, it is possible to detect a change in infrared radiation from a human body by closing and opening the shutter even though the human does not move. Therefore, the device disclosed in the document 1 can avoid a lighting load being turned off, because it can detect the presence of a human even if the human does not move.
However, the device of the document 1 cannot distinctively detect a simple heat source (object) and a human body. Accordingly even if a heat source other than a human body is present in a sensor range of the pyroelectric element, the On state of the lighting load is maintained. For example, if a human moves in the sensor range to turn on a heat source such as a TV set, a desk lamp or the like and then moves out of the sensor range, the heat source remains in the sensor range. Accordingly, if the heat source is not removed, the pyroelectric element continues detecting the heat source to keep the lighting load turned on.